The present invention relates to an apparatus for drying bulk materials comprising a substantially cylindrical bulk material hopper with a funnel-shaped lower end, a bulk material inlet at the top of the hopper, a bulk material discharge device at the bottom of the hopper, an air inlet for introducing heated dry air for drying bulk material in the hopper, and an air outlet for discharging moisture-containing air; especially an apparatus in which the air inlet and air outlet of the hopper are connected in a closed air circuit including an air drying system for drying and heating moisture-laden air from the hopper, an exhaust air duct leading from the air outlet of the hopper to the air drying system, an air supply duct leading from the air drying system to the air inlet of the hopper, and a blower for circulating heated, dry air from said air drying system through bulk material in the hopper and moisture laden air from the hopper back to the air drying system.
European patent application No. EP 95,265 discloses an apparatus for controlling the process of drying bulk materials, in which heated air introduced into the material to dry it is analyzed with regard to its temperature and its moisture content, and the exhaust air is also analyzed for its temperature and its moisture content. A temperature sensor and a moisture sensor also are placed within the bulk material, which furnish information on the condition of the material. All of the measured values are sent to a processor which controls the drying process in accordance with the measured data. The combination of a temperature sensor and a moisture sensor is very costly. Also, it is known that the moisture sensors commonly used are very inaccurate. Particularly when the bulk material to be dried is plastic granules, this kind of drying control is inaccurate, since the drying of plastics involves the removal of a very slight residual moisture which is very
difficult to measure.
German published patent application No. DE 2,052,334 discloses a method and an apparatus for drying bulk materials in which moisture is removed from the bulk material in a hopper with a heated dry gas, especially air. In order to adequately remove the moisture from a large amount of material, it is proposed to feed additional heat to the hopper. This, however, requires an additional air circuit as well as a heating system to be disposed therein, and this additional air circuit is activated when the temperature of the exhaust air falls below a predetermined level.
Normally, the bulk material dried in the hopper is removed more or less continually, and fresh bulk material corresponding to the amount removed is introduced at the top. As long as the amount of material removed is substantially constant, the heated air fed in can be adapted to this throughput of bulk material. If, however, the bulk material throughput is increased to the extent that drying with the maximum available air input is no longer sufficient, there is a danger that bulk material will leave the drying hopper with an excessively high moisture content, so that high quality further processing of this bulk material will no longer be possible. No solution of this problem can be found in the known state of the art.